Experimental observation of quantum hall effect, Berry's phase and many-body effects in graphene

Philip Kim

Physics Department, Columbia University, New York, USA

Graphene, an isolated single atomic layer of graphite, is an ideal realization of a 2-dimensional (2D) system. Its electrical behavior is, however, expected to differ dramatically from the well-studied case of quantum wells in conventional semiconductor interfaces. This difference arises from the unique electronic properties of graphene, which exhibits electron-hole degeneracy and vanishing carrier mass near the point of charge neutrality. Recent advances in micromechanical extraction and fabrication techniques for graphite structures now permit such exotic 2D electron systems to be probed experimentally. In this presentation, we will discuss our recent experimental investigation of magneto transport in a high mobility single layer of graphene. Adjusting the chemical potential using the electric field effect, we observe an unusual half integer quantum Hall effect (QHE) for both electron and hole carriers in graphene. The relevance of Berry's phase to these experiments is confirmed by magneto-oscillations, which is a consequence of the exceptional topology of the graphene band structure. In addition, we report the observation of splittings of Landau levels (LLs) under high magnetic field up to 45 T. We discover the n=0 LL splits into four sublevels, lifting spin and sublattice degeneracy, potentially indicating a many-body correlation in this LL. In the higher LL, only the spin degeneracy seems to be lifted within experimentally accessible high magnetic field. The effective g-factors were found close to the bare electron g-factor.